1. Field of the Invention
The invention relates to coated halogen bleach particles and a method for bleaching substrates through slow uniform release of active halogenating agent from the particles.
2. The Prior Art
Particles containing oxidants for bleaching substrates have been widely disclosed in the literature. Much research has focused upon coating or encapsulating chlorinating agents, e.g. dichloroisocyanurate granules, to obtain delayed, slow release of active oxidant.
When used for cleaning clothes in automatic washing machines, several problems are noted with encapsulated oxidants. Low bleaching strength is encountered because of incomplete dissolution of the encapsulates during the standard wash cycle. Another problem is severe fabric color damage from the localization of released bleach. Generally, bleaching products are placed into the automatic washing machine simultaneously with the dry load. Bleach and fabric remain in close contact as the machine fills with water. Local high concentrations of bleaching actives thereby come into contact with fabric surfaces. Under these conditions, very small spots resembling pinholes appear on the fabric.
U.S. Pat. No. 4,136,052 (Mazzola) reports to have solved the pinhole problem caused by localized high concentrations of bleach. The patent provides a special coating which encapsulates the bleaching compound. An active chlorinating agent is surrounded by a first non-reactive coating combination of fatty acid and wax. A second time controlled coating is applied containing fatty acid with a material exhibiting inverse aqueous solubility with respect to temperature. The outer, second coating is more resistant to dissolution in hot than in cold water. By this means, sufficient delayed release is provided in hot water to prevent pinholing.
U.S. Pat. No. 3,908,045 (Alterman et al.) discloses dichloroisocyanurate salts encapsulated with a first coating of a saturated fatty acid surrounded by a second coating of soap. The latter coating is formed by treatment of portions of the inner fatty acid coating with a solution of an alkali metal hydroxide.
The prior art compositions of soap coated chlorine bleach provide adequate protection against pinhole type fabric damage only at low and medium wash temperatures. Unfortunately, at hot wash temperatures, pinholing is still a problem. It has been suggested that hot water pinholing results from non-uniform coating, fabric damage being caused by the inadequately encapsulated particle fractions. Uniformly coated particles have, so far, been unobtainable. To solve the problem, average coating weights have been increased by as much as 50% over the known art. Even these increased thicknesses do not ensure complete absence of pinholing at hot wash temperatures. Very thick coatings, which do control pinholing, are deficient because they hinder chlorine release at low wash temperatures and afford no bleaching.
Consequently, it is an object of the present invention to provide bleach particles which eliminate pinholing yet have satisfactory active halogen release at all wash temperatures.
A further object of this invention is to provide bleach particles that do not release active halogen oxidant during the water fill cycle of an automatic washing machine but subsequently completely release active oxidant within the wash cycle.
Another object of this invention is to provide a method for bleaching a variety of flexible or hard surfaced substrates.